Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

Ultrahigh strength and low yield ratio of niobium-microalloyed 900 MPa pipeline steel with nano/ultrafine bainitic lath

Abstract: An ultra-low carbon niobium-microalloyed steel with yield strength of ∼900 MPa has been processed on a pilot-plant scale. The microstructure of the steel is primarily characterized by lower bainite and acicular ferrite, with small fraction of lath-martensite and martensite–austenite (MA) constituents. Bainite is present as fine domains. A combination of niobium and titanium precipitates was observed at the grain boundaries and in the interior of the grains and includes irregular (∼40–150 nm of (Nb, Ti)(C, N)) and fine cuboidal/spherical particles of NbC (∼30–50 nm). It was observed that accelerated cooling inhibited the precipitation of Nb and Ti carbides. The Charpy impact toughness at −20 °C was 200 J and tensile elongation was 15% with the yield ratio of less than 0.84. The good matching of high strength and low yield ratio was realized by two-stage thermo-mechanical rolling combined with fast cooling.

Effect of microstructure on the impact toughness of Nb-microalloyed steel: Generalisation of existing relations from ferrite–pearlite to high strength microstructures

Abstract: The present work relies on the production of selected microstructures through the application of thermal and thermomechanical laboratory tests, followed by mechanical testing and microstructural characterisation. The relations that link the microstructure parameters and the tensile properties have already been discussed and extended from ferrite–pearlite to high strength microstructures in a previous work. Using these results as a starting point, the present work goes a step forward and develops a methodology to consistently incorporate the effect of mesotexture (EBSD) into the existing relations that link the Charpy impact toughness to the microstructure. The result is an extension of the existing equation for the FATT from ferrite–pearlite to high strength microstructures (bainite, tempered martensite). The upper shelf energy for the Nb-microalloyed steel under consideration correlates linearly with the sum of the different terms of the FATT equation (solutes, grain boundary carbides, pearlite and precipitation/dislocations strengthening) excepting the grain size.

Effect of the bainitic transformation temperature on retained austenite fraction and stability in Ti microalloyed TRIP steels

Abstract: TRIP properties in four hot-rolled microalloyed steels were studied as a function of bainitic domain entrance temperature. It was found that this temperature affects the size and morphology of the retained austenite grains. Size and morphology was shown to play a role on the stability of the retained austenite. This stability was shown to be the major factor affecting the mechanical properties of these steels. In the case of the lowest bainitic temperature, the TRIP effect is hindered due to the premature transformation of austenite into martensite probably due to an insufficient carbon enrichment of the small austenite grains. Moreover the bainitic treatment performed at the highest temperature contained a very low volume fraction of retained austenite that does not transform even under deformation. Using TEM and the Kikuchi-line method, it was shown that the sample that did not undergo a phase transformation during FIB milling had a carbon concentration in its retained austenite grains of approximately 1.7%.